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LM70880: Webench Fail

SHINYOUNG KIM
Prodigy 40 points
Part Number: LM70880
Other Parts Discussed in Thread: LM70840

Tool/software:

Dear TI

I am designing a circuit using a LM70880 with the following specifications.

   Vin: 32 ~ 80V

   Vin(nom): 48V

   Vout: 24V

   Iout(full load): 4A

I tried to get a reference circuit using Webench, but it didn't generate the circuit due to stability issues.

After that, I ran Webench with the above conditions without specifying the IC(LM70880).

At this time, the LM70840 circuit was generated.

Will it be difficult to use LM70880 with the above conditions?

Thanks,

Best regards,

Shinyoung Kim

  • 0
    TI__Guru 56345 points

    Hi Shinyoung,

    Please use our quickstart calculator which provides similar performance and analysis as the WEBENCH model. You can find the quickstart calculator below.

    https://dr-download.ti.com/design-tools-simulation/calculation-tool/MD-LuIrFjvWzJ/A1/LM70XX0_quickstart_calculator_A1.xlsm

    Ben

  • 0 in reply to Ben C
    Prodigy 40 points

    Hi Ben

    Thank you for providing newer version quickstart calculator.

    I had a A0 version early. 

    I found that the power disiipation calculation results of the IC are significantly different between A0 and A1 versions.

    I will design a circuit using the A1 version document.

    I have a few more questions.
    1. When selecting the crossover frequency, is there any problem if it is smaller than 1/10 of the switching frequency?
    2. What is the approximate stability range of phase margin and gain margin?
     
    Thanks,
    Best regards,
    Shinyoung Kim
  • 0 in reply to SHINYOUNG KIM
    TI__Guru 56345 points

    Hi Shinyoung,

    SHINYOUNG KIM said:
    I found that the power disiipation calculation results of the IC are significantly different between A0 and A1 versions.

    The curves in the efficiency graph are different between A0 and A1. Maybe that is what you are referring to.

    SHINYOUNG KIM said:
    When selecting the crossover frequency, is there any problem if it is smaller than 1/10 of the switching frequency?

    When you are saying smaller than 1/10 of the switching frequency, do you mean like 1/20 of the switching frequency or like 1/5 of the switching frequency?

    SHINYOUNG KIM said:
    What is the approximate stability range of phase margin and gain margin?

    Usually, I like to design greater than 50 degrees of phase margin. Anything lower will cause the loop to behave a little unstable (ringing seen at the output after a load transient event). Do not go below 45 degrees or the loop will be unstable. Below is a link that describes the stability of the loop as phase margin changes.

    https://www.ti.com/lit/an/slva381b/slva381b.pdf

    For gain margin, a minimum of 6dB is required while greater than10dB is considered optimal when the phase margin curve crosses 0 degrees. Below is a pdf that describes in detail on loop stability.

    0066.COMPENSATION.pdf0066.COMPENSATION.pdf

    Ben

  • 0 in reply to Ben C
    Prodigy 40 points

    Hi Ben,

    1. After checking again, I found that when I changed the parameters related to the Schottky diode in the A0 version document to 0, the power dissipation of the IC became the same as that of A1.

    2. It means 1/20.

    3. Thank you for your suggestion and explanation.

    Thanks,

    Best regards,

    Shinyoung Kim

  • 0 in reply to SHINYOUNG KIM
    TI__Guru 56345 points

    Hi Shinyoung,

    SHINYOUNG KIM said:
    1. After checking again, I found that when I changed the parameters related to the Schottky diode in the A0 version document to 0, the power dissipation of the IC became the same as that of A1.

    The schottky diode entry was an oversight that was carried over from a previous product. As you noticed, A1 removed the schottky diode entry.

    SHINYOUNG KIM said:
    2. It means 1/20.

    The loop response will be slower. You will see more over and undershoot when a load transient event occurs.

    SHINYOUNG KIM said:

    3. Thank you for your suggestion and explanation.

    Thanks,

    No probelm! :-)

    If there are no further questions, please close the thread by clicking on "resolved". Thank you and good luck to you Shinyoung.

    Ben

  • 0 in reply to Ben C
    Prodigy 40 points

    Hi Ben,

    I have one more question. If I add an external Schottky diode to the SW node, will it have any effect on power efficiency?

    Thanks,

    Best regards,

    Shinyoung Kim.

  • 0 in reply to SHINYOUNG KIM
    TI__Guru 56345 points

    Hi Shinyoung,

    Yes, the efficiency may increase slightly with a schottky diode but it will add to your total overall BOM and cost. Something to think about...

    Ben 

  • 0 in reply to Ben C
    Prodigy 40 points

    Hi Ben,

    Thank you. I will think about it.

    Thanks

    Shinyoung KIm.